This invention is in the field of optical sensors. Optical sensors for sensing position and other physical properties are of great interest in the aerospace industry. They can be used in fly-by-light systems on aircraft to sense position of actuators that control flaps, rudders, and the like. In this environment, optical sensors are particularly attractive because the sensors and the optical fibers that transmit signals to and from them, are lightweight, compact and immune to EMI effects. Examples of optical sensors are found in U.S. Pat. Nos. 4,740,688, 4,833,317, 4,931,636, 4,964,727, and 5,068,528, all of which are incorporated herein by reference.
Wavelength division multiplexing (WDM) techniques come into play in many proposed optical sensors. U.S. Pat. Nos. 4,740,688, 4,931,636, 4,964,727, and 5,068,528, referenced above, show examples of such systems. FIG. 1 shows schematically a typical WDM sensor system. A typical WDM system includes a broadband light source 10, a WDM transducer 12, and a demultiplexing receiver 14. Typical broadband sources for this application include multiple LED's with their outputs combined by a coupler, multiple LED's fabricated on the same die, and incandescent light bulbs. FIG. 2a shows the typical relationship between intensity and wavelength for such broadband sources. The transducer 12 selectively modulates different wavelength bands in a manner which corresponds to the property being sensed. One example of a WDM transducer is an encoder plate which either rotates or moves linearly in response to movement of the element whose position is being sensed. In a transducer with a multitrack digital code plate, the light in selected wavebands is modulated digitally (on or off). An example of such a transducer is shown in U. S. Pat. No. 4,740,688. FIG. 2b shows an example of a possible output from such a transducer in which the light intensity in different wavelength ranges varies with the position of the encoder plate. Alternatively, the light intensity can be analog modulated by a code plate whose density varies with position, or by microbend induced losses in a fiber. In an intensity modulated transducer, it is necessary to provide an intensity reference, typically, light at a second wavelength which passes through the transducer unmodulated. An example of such a system using a code plate is shown in U.S. Pat. No. 4,931,636. A system using micobend induced losses is shown in U.S. Pat. No. 5,020,379, which is incorporated herein by reference.
The demultiplexing receiver 14 includes a dispersion element 16 which separates light into the different wavelength bands, and an array of photodetectors 17 where each pixel measures the light intensity in a particular wavelength band. The, outputs of the detectors are then sequentially read as a serial bit stream, or as a parallel data word.
In a two wavelength intensity referencing system the demultiplexing receiver can be a simple WDM splitter and two PIN diode detectors. However, systems where up to twenty wavelength bands are modulated may have important performance advantages for multiplexing transducers. But the demultiplexing receiver for such a multi-wavelength system requires a dispersion device such as a diffraction grating, an array detector such as a PIN diode or CCD array, and associated optical components such as lenses. The complexity of such a demultiplexing receiver makes it difficult to fabricate, hard to maintain performance over temperature, and therefore expensive. Such a receiver also suffers from being optically inefficient, with overall efficiencies of less than 10% being typical.
What is needed then, is an optical sensor system that is relatively simple, easy to fabricate, optically efficient, and reliable in its operating environment.